1. Field of the Invention
The present invention relates to a magnetic recording medium, especially to a magnetic recording medium which is excellent in both frequency characteristics and surface properties. More particularly, it relates to a magnetic recording medium which utilizes a vinyl chloride-vinyl acetate copolymer containing at least 10% of vinyl alcohol, a urethane resin and a polyester resin as binding components in a magnetic layer.
2. Description of the Prior Art
Generally speaking, a magnetic recording medium is produced by coating a substrate such as a film of e.g. an acetate, PVC, polyester etc. with a magnetic paint having a magnetic powder such as .gamma.-Fe.sub.2 O.sub.3 etc. dispersed in a polymer resin binder to form a magnetic layer thereon. Recently, with the expansion of uses of such magnetic recording media, further improved characteristics have been increasingly required of them. For example, in the recent magnetic recording media, especially cassette tapes for audio components, video tapes etc., those having more excellent frequency characteristics have been demanded. In order to comply with such a demand, it is generally practiced to enhance the dispersing properties of the magnetic particles in the magnetic layer and the surface properties of the magnetic layer of the magnetic recording medium. In other words, it is necessary to uniformly disperse the magnetic particles in the magnetic layer, reduce spacing loss by enhancing the surface properties of the magnetic layer, impart appropriate softness to the magnetic layer so as to improve the touch with a head, and so forth. On the other hand, in the magnetic powder itself, it is necessary to improve the coercive force, increase the saturation magnetization instensity (Is), reduce the magnetic particles in size, and so forth. However, such improved particles have a tendency that the dispersibility in the magnetic layer is lowered. Therefore, various kinds of vinyl chloride type polymers have heretofore been proposed as resin binders so as to disperse a magnetic powder, but since even such vinyl chloride type polymers have a disadvantage when used alone, combinations of a vinyl chloride-vinyl acetate copolymer with a urethane resin are being generally employed at present. However, the vinyl chloride-vinyl acetate copolymer has a disadvantage; since it has a high glass transition point and hence it is difficult to work on the surface, the surface properties are poor, whereas the urethane resin, which is soft and hence has an excellent touch with a head and also has a low glass transition point and hence is good in surface workability, suffers from a disadvantage that the dispersibility of the magnetic particles is less favorable as compared with the vinyl chloride-vinyl acetate copolymer resin. Therefore, the present situation is such that even a combination of a vinyl chloride-vinyl acetate copolymer and a urethane resin cannot present entirely satisfactory results. Under such circumstances, as a means for improvement, the present applicant once proposed a magnetic recording medium which utilizes a binder system comprising a combination of a vinyl chloride-vinyl acetate copolymer containing at least 10% of vinyl alcohol as a binder component for a magnetic layer, which was disclosed as Japanese Patent Application Laid-open No. 55-70936. As stated in that patent application, it was discovered that the addition of a small proportion of a vinyl chloride-vinyl acetate copolymer containing at least 10% of vinyl alcohol enhances the dispersibility of a magnetic powder. However, in that resin binder system, the surface properties of the magnetic coat were not completely satisfactory since the surface workability was not so good owing to the raise of the glass transition point (Tg).
In view of the above point, the present inventors have intensively studied on vinyl chloride-vinyl acetate copolymer based resin binders and, as a result, have discovered that the use of a binder obtained by adding a urethane resin to a vinyl chloride-vinyl acetate copolymer containing at least 10% of polyvinyl alcohol to plasticize it and further adding thereto a polyester resin can provide a smooth magnetic coat which has a high coercive force with a high saturation magnetization intensity and further which has finely divided magnetic particles uniformly dispersed. The present invention has been achieved based on this discovery, and it provides a magnetic recording medium, the magnetic coat of which has excellent surface properties without reducing the dispersibility of the magnetic particles in the magnetic layer.